Recorded some tracks on Garageband on the Mac; and want to transfer the file to my iOS device.
You can only sync GarageBand projects from your mac to the iPad that originally have been created on your iPad. Creating a project on the iPad, then sending it to a mac, opening it on the Mac, doing some edits (but nothing that is not supported on the iPad), then upload it again to the iPad will work. But sendind a Mac-created GarageBand project to the iPad is not possible. The applications are just too different.
You can open your mp3 in GarageBand on the iPad, however. Sync it to the iPad using iTunes, so you see it in the Music.app on your iPad. All songs there can be accessed from the Loop Browser on the iPad, see: Import a song from the Music app
To sync projects between iPad and Mac use iTunes.
To send a song from the iPad to your mac as a project, share the song to iTunes > GarageBand.
Sync your iPad with your computer.
In iTunes, the exported song appears in the Documents list in the File Sharing area when GarageBand is selected.
To send the project back to the iPad, add it to garageBand's Documents list in iTunes.
Regards
Léonie